Currencies that are traded internationally tend to influence the world economics. The US Dollar being one of the international currencies is also highly influenced by the social, political, and economic changes that happen around the world. The key factors that influence the movement of the US Dollar are the demand for US Dollar compared to the availability of the US Dollar to people. There is also the factor of emotional buying or selling. Traders across the globe usually perceive the USA as a very stable economy, however if they feel that America is becoming unstable due to whatever reasons, then they would start selling their investments in the US Dollar and move to other currencies which are seen as more stable.

An example of such a situation could be the unemployment rate in the US. If it is low, people have more confidence in the US economy and will invest in US Dollars. Meanwhile, if the unemployment rate is increasing, people become finicky and start selling their investments in the US.

The supply and demand of oil and gold also influence the US Dollar. Using the Portfolio Prophet can help you see this more clear. There are also situations when certain market players will tend to create an artificial situation. They either push more money into the market or buy more US Dollars from the market, thus varying the rate of US Dollar. This is a never ending cycle of movement for the US dollar which makes trading it a bit more aggressive.